Production of semiconductor devices requires formation of finer lithography patterns for increasing the degree of integration. For forming lithographic micropatterns, use of a radiation (light) source of short wavelength (as used herein, “light” refers to a type of radiation) is a key technique. In addition to g-rays and i-rays, which have hitherto been used, far-ultraviolet radiations such as a krypton fluoride (KrF) excimer laser radiation (wavelength: 248 nm) and an argon fluoride (ArF) excimer laser radiation (wavelength: 193 nm) have been introduced for mass production of semiconductor devices. Meanwhile, studies have been conducted on lithography techniques employing, as a radiation source, a fluorine dimer (F2) excimer laser radiation (157 nm), an extreme ultraviolet radiation (EUV), an electron beam (EB), or the like.
Such a lithography technique suitably uses a positive chemical amplification-type copolymer for lithography. Such a copolymer has a repeating unit having a structure including a polar group that can be dissolved in an alkaline developer (hereinafter the group may be referred to as an “alkali-soluble group”), the alkali-soluble group being protected by a substituent group that is unstable to acid and inhibits solubility in an alkaline developer (hereinafter the group may be referred to as an “acid-dissociable dissolution-inhibitive group”); and a repeating unit having a polar group for enhancing the adhesion of the copolymer to, for example, a semiconductor substrate, or regulating the solubility of the copolymer in a solvent for lithography or an alkaline developer.
For example, as has been known, a lithography technique using KrF excimer laser as an exposure light source employs, for example, a copolymer having a repeating unit derived from hydroxystyrene; and a repeating unit in which a phenolic hydroxyl group derived from hydroxystyrene is protected by an acid-dissociable dissolution-inhibitive group (e.g., an acetal structure or a tertiory hydrocarbon group), or a repeating unit in which a carboxyl group derived from (α-alkyl)acrylate is protected by an acid-dissociable dissolution-inhibitive group (e.g., an acetal structure or a quaternary hydrocarbon group) (see, for example, Patent Documents 1 to 4). There has also been known a copolymer having a repeating unit in which an alicyclic hydrocarbon group serves as an acid-dissociable dissolution-inhibitive group for increasing the dry etching resistance of the copolymer or the difference in rate of dissolution of the copolymer in an alkaline developer between before and after light exposure (see, for example, Patent Documents 5 and 6).
In a lithography technique using, for example, ArF excimer laser of shorter wavelength as an exposure light source, attempts have been made to employ a copolymer which does not have a hydroxystyrene-derived repeating unit exhibiting high absorption coefficient with respect to a wavelength of 193 nm. As has been known, such a lithography technique employs a copolymer having a repeating unit having a lactone structure as a polar group for enhancing the adhesion of the copolymer to, for example, a semiconductor substrate, or regulating the solubility of the copolymer in a solvent for lithography or an alkaline developer (see, for example, Patent Documents 7 to 10).
In recent years, immersion lithography has been proposed. This is a technique in which light exposure is carried out by introducing, between an objective lens and a lithography thin film, a liquid exhibiting a refractive index higher than that of air (e.g., water). As compared with the case of a conventional lithography technique in which an air layer is present between an objective lens and a thin film (hereinafter the technique may be referred to as “dry lithography”), immersion lithography enables finer lithography patterns to be formed by means of a light source having the same wavelength as that of a light source used in dry lithography, since the numerical aperture of a lens can be increased even when a light source of the same wavelength is employed, and the depth of focus can be increased even when a lens of the same numerical aperture is employed. Therefore, studies have been actively conducted on immersion lithography employing ArF excimer laser as a light source and, in particular, water as an immersion liquid, so that the lithography can be put into practice as a next-generation lithography technique. The copolymer proposed to be employed in such ArF immersion lithography is the same as that known in conventional ArF dry lithography techniques (see, for example, Patent Documents 11 to 13).
However, the aforementioned known copolymers may raise problems, due to its high water wettability. Specifically, when water is introduced between an objective lens and a film which is formed of the copolymer and is exposed to light, after the objective lens has been moved on the film together with water, water droplets remain on the film, which may cause a pattern defect called “water mark.” In addition, the remaining water droplets may enter the film to elute an additive such as a radiation-sensitive acid-generator, causing variation in sensitivity or abnormal patterning.
Meanwhile, for reduction of problematic variations in line width of lithography patterns during formation of fine patterns, there has been proposed a copolymer having a structure in which an alcoholic hydroxyl group derived from a hydroxycycloalkyl (α-lower alkyl)acrylate is protected by an alkoxyalkyl group (Patent Document 14). The copolymer disclosed in this patent document has a particularly preferred structure in which an alcoholic hydroxyl group is protected by an acetal bond, and only such a structure is described in examples of the document. However, such a structure is very unstable to water or heat, and cannot be used in immersion lithography.
Thus, there has never been known a copolymer which is suitable for lithography employing ArF excimer laser as an exposure light source, which has a repeating unit having a structure in which an alkali-soluble group is protected by an acid-dissociable dissolution-inhibitive group and a repeating unit having a lactone structure, and which exhibits excellent surface characteristics suitable for immersion lithography (e.g., water removability or water repellency represented by the falling angle or sweepback angle of water).    Patent Document 1: JP-A-1984-045439    Patent Document 2: JP-A-1993-113667    Patent Document 3: JP-A-1998-026828    Patent Document 4: JP-A-1987-115440    Patent Document 5: JP-A-1997-073173    Patent Document 6: JP-A-1998-161313    Patent Document 7: JP-A-1997-090637    Patent Document 8: JP-A-1998-207069    Patent Document 9: JP-A-2000-026446    Patent Document 10: JP-A-2001-242627    Patent Document 11: JP-A-2005-227332    Patent Document 12: JP-A-2005-234015    Patent Document 13: JP-A-2005-316259    Patent Document 14: JP-A-2006-106497